JavaScript examples

Copy ready-to-use JavaScript snippets for functions, from returning a response to reading cookies and denying requests on a firewall.

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Copy any of these JavaScript snippets into a function as a starting point for your own code. The examples cover the patterns functions are most often used for: returning a response or a JSON payload, redirecting traffic, reading cookies, running an A/B test, and denying a request by country. They are grouped by where the function runs: Applications, for functions that handle the fetch event on an application, and Firewall, for functions that handle the firewall event on a firewall.

Most handlers in this collection are written in the Service Worker pattern, which registers the handler with addEventListener. New code uses ES Modules, and Process a request body is written that way: refer to Best practices for the current pattern, and to Migrate handler patterns in Functions for the before-and-after of each handler.

Applications

A/B testing

Split traffic between two responses in a function, picking a group at random and recording it in a cookie.

Extract a cookie value

Read a named cookie from the request header in a function and use its value to shape the response you return.

Hello world

Return a fixed string from a function, without contacting an origin, and confirm the function is deployed and serving traffic.

Process a request body

Read the body a client sends, switching on the Content-Type header to parse JSON, form data, text, binary, and streamed payloads.

Redirect all requests to one URL

Send every incoming request to one destination URL with a 301 redirect returned by a function, before it reaches your origin.

Respond with another site

Answer a request with content fetched from another site, relaying the upstream response while the browser keeps the original URL.

REST APIs

Call a REST API from a function with fetch, using the GET, DELETE, POST, PUT, and PATCH methods with JSON data.

Return HTML

Build an HTML page inside a function and return it with the correct content-type header, without rendering it on an origin server.

Return JSON

Serialize an object and return it as a JSON response with the correct content-type header, for lightweight APIs and middleware.

Using args

Pass JSON Args into a function from its instance and read them at runtime to change behavior without editing code.

Firewall

Add a response header

Add a response header based on the visitor’s country, with a function that runs on a firewall and reads GeoIP metadata.

Deny a request by country

Block requests from a chosen country before they reach your application, with a function that runs on a firewall.

Functions on a firewall

Deny, drop, modify, or answer a request from one firewall handler, using the event actions available to a function on a firewall.

Quickstart

Take one of these snippets from the editor to a URI that answers it.

Handlers

Read the reference for the fetch and firewall handlers and the parameters each one receives.

Best practices

Scope, configure, and instrument a function so it stays inside the ceilings that bound every invocation.

Functions limits

Review the default limits for functions and function instances, including code size, memory, and execution time.

Troubleshooting

Find the cause when a function never runs, stops before it responds, or produces no log output.